1. Field of Invention
This invention relates to carbon nanotubes and image display devices using carbon nanotubes.
2. Description of Related Art
Nanotechnology is the study of devices that operate at the nanometer (10xe2x88x929 m) scale. The possible uses for this technology are wide-ranging, from medical applications to various application in computing and consumer electronics. However, these structures present challenging technical issues, such as how to control them, and how to construct them by using macroscopic design principles in a realm in which classical approaches are not always useful.
Carbon nanotubes are a recent development in the field of nanotechnology. Carbon nanotubes, like diamond or graphite, are a variant from of crystalline carbon, structurally related to the carbon fullerene xe2x80x9cbuckyballxe2x80x9d or C60. Instead of a ball shape, the carbon nanotube structure takes the form of a long tube of graphene hexagons capped at each end by a fullerene hemisphere. The nanotubes can be either single or multi-walled. Multi-walled tubes contain multiple layers of concentric tubes.
The conductive properties of nanotubes depend on the exact helical structure of the nanotubes. The three groupings of helical structure of carbon nanotubes are xe2x80x9carmchairxe2x80x9d, xe2x80x9czigzagxe2x80x9d and xe2x80x9cchiralxe2x80x9d. Armchair nanotubes are metallic. Zigzag and chiral nanotubes can be either semiconducting or metallic, depending on the particular properties of its lattice parameters. A single-walled carbon nanotube typically has a width of 1.2 to 1.4 nm and a length of up to 10 xcexcm, depending on production and purification methods.
In order to better understand the properties of nanotubes, it is helpful to look at the properties of a graphene sheet and imagine a segment of it rolled into a tube. FIG. 1 shows a segment 10 of a graphene sheet. The nanotube is made by connecting vertices 20 on one of the dotted lines shown in FIG. 1 with its counterpart on the other dotted line. The direction of the tube axis T along the segment 10 determines the conductive properties of the nanotube. The direction of the tube axis T is usually described in terms of a chiral vector C that is perpedicular to the tube axis T. The chiral vector C is typically written as (n,m), where n and m are integers. The integers n and m are related to the chiral vector C as follows:
C=na1+ma2xe2x80x83xe2x80x83(1) 
where:
a1 and a2 are the two lattice unit vectors.
The parameters n and m determine the width and helicity of the nanotube. Further, a tube is metallic if nxe2x88x92m is a multiple of three, otherwise it is semiconducting.
FIG. 2 shows the relationship between armchair, zigzag and chiral tubes. For armchair tubes, the chiral vector Ca is described as (n,n). For zigzag tubes, the chiral vector Cz is described as (n,0). All other tubes are chiral tubes.
The conductive properties of nanotubes arise from the unique hexagonal structure of graphene. Twisting or bending a tube can change its electrical properties from metallic to semiconducting, or vice-versa. Also, applying a magnetic field parallel to a nanotube axis can change its conduction properties.
Nanotubes, like diamond and in-plane graphene, are very strong and resist pressure well. A nanotube can absorb large amounts of force without bending, and after a critical amount of force is applied, the nanotube bends rather than shatters.
Nanotubes were first produced by using a carbon anode in an electrical arc that vaporized the carbon and deposited the tubes as a mass of tangled ropes. Other processes have been developed, such as laser ablation, most of which use catalysts such as Ni. Currently, no regular means exists for controlling the length or the helical properties of nanotubes produced using these methods. However, it has been shown that using catalysts can enhance production of single-walled versus multi-walled tubes.
For many applications, the ability to form regular arrays of nanotubes on a substrate is required. Shoushan Fan et al., xe2x80x9cSelf-Oriented Regular Arrays of Carbon Nanotubes and Their Field Emission Propertiesxe2x80x9d, Science, pp. 512-514 (1999) describes a method for forming arrays of nanotubes on a substrate. This method includes etching holes in a Si substrate and using a Fe mask to create a pattern. The nanotubes are then grown using chemical vapor deposition (CVD), and bundles of tubes align themselves in a regular pattern. A similar method, presented in Z. F. Ren et al., xe2x80x9cSynthesis of Large Arrays of Well-Aligned Carbon Nanotubes on Glassxe2x80x9d, Science, pp. 1105-1107 (1998), involves using acetylene and ammonia, and a glass substrate covered with a layer of nickel catalyst.
The size and unique properties of carbon nanotubes make them ideal candidates for use in many different nanotechnology applications. Much of the current work on nanotubes focuses on production methods and the use of nanotubes as nanometer-sized wires and uniform field-emission devices. For example, U.S. Pat. No. 6,019,656 describes a cold-cathode field emitter method for use in a display device. This device uses a voltage differential to cause an electron field emission from the array of nanotubes.
Other methods have been developed that generate a current in a nanotube or nanotube array. For example, a current can be generated in a nanotube by a pair of laser beams, wherein the wavelength of one laser beam is double the other. Altering the relative phase of the two beams controls the direction of the induced current. This approach is presented in Meale, E. J., xe2x80x9cCoherent Control of Photocurrents in Graphene and Carbon Nanotubesxe2x80x9d, Physical Review B, p. 61(2000). Also, a current can be generated in a nanotube by injecting carriers into the nanotube from a set of contacts.
The various exemplary embodiments of the method and apparatus according to this invention uses light beams to create a current in at least one nanotube in an array of nanotubes. The concepts introduced by this invention helps to bridge the gap between the macroscopic scale and the mesoscopic scale in that it presents a useful alternative to fabricating contacts to the nanotubes which can be easily used by macroscopic devices. The macroscopic device can convert electronic information to light, which can then be used to communicate with the mesoscopic world.
According to one embodiment of the invention, a nanotube assembly has a plurality of nanotubes arranged in an array. An optical excitation device is provided adjacent to the nanotube assembly. The optical excitation device illuminates at least one of the plurality of nanotubes such that electrons are emitted from the at least one nanotube.
According to another aspect of this invention, the optical excitation device includes a diffraction grating and a piezoelectric crystal disposed adjacent to the diffraction grating. A radiation source generates a write beam incident to the piezoelectric crystal, a read beam incident to the diffraction grating, and an erase beam incident to the diffraction grating. When voltage is applied to the piezoelectric crystal, the write beam scans across the diffraction grating and forms a grating pattern in the diffraction grating. The read beam reads the grating pattern as a holographic image on the at least one nanotube. The erase beam erases the grating pattern.
According to another aspect of this invention, the diffraction grating includes a first layer, the first layer being provided with a plurality of injection elements. The diffraction grating also includes a second layer provided over the first layer, the second layer being provided with a plurality of scattering elements. The first layer and the second layer are made of semiconductor material. The image injecting elements are made of doped n-type semiconductor material and the scattering elements are made of heavily doped p-type semiconductor material.
According to another aspect of this invention, the plurality of injecting elements have an electronic band gap greater than the photon energy of the read beam such that the injection elements are transparent to the read beam.
According to another aspect of this invention, the plurality of scattering elements have an electronic band gap less than or equal to the photon energy of the read beam such that the scattering elements are opaque to the read beam.
According to another aspect of this invention, the first layer and the second layer have an electronic band gap greater than the photon energy of the read beam such that the first layer and the second layer are transparent to the read beam.
According to another aspect of this invention, the injecting elements have an electronic band gap less than the photon energy of the write beam such that the injecting elements are opaque to the write beam.
Other embodiments provide a method of displaying an image, including illuminating at least one of a plurality of nanotubes arranged in an array of nanotubes such that electrons are emitted from the at least one nanotube. The various exemplary embodiments of the methods according to this invention includes generating a write beam incident to a piezoelectric crystal and a read beam incident to a diffraction grating such that, when voltage is applied to the piezoelectric crystal, the write beam scans across the diffraction grating and forms a grating pattern in the diffraction grating and the read beam reads the grating pattern as a holographic image on the at least one nanotube.
These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the systems and methods according to this invention.